1. Field of the Invention
This invention relates to a flow control valve used in a fluid circuit for intentionally moving or driving fluid equipment, and more particularly to a magnetic flow control valve capable of continuously changing the flow rate or the pressure of a fluid by controlling the force for shifting a valve body toward a valve seat when required.
2. Description of the Prior Art
Flow control valves are used to adjust the flow rate in fluid circuits so as to regulate, for example, moving speeds of actuators or rates in pressure increase. Flow control valves are generally required in performance to have capabilities of flow rate adjustment in wide ranges and fine adjustment and stable and continuous characteristics.
A needle valve is the most commonly used flow control valve. It includes a needle valve member having a conical end adapted to be moved toward and away from a valve seat with the aid of a screw-threaded rod to change open degree of the valve. However, it encounters many problems due to its construction. For example, it is very difficult to eliminate plays or clearances between screw-threads and misalignment of axes of an orifice and the needle valve body. Moreover, a relationship between rotated angles of the screw-threaded rod and opening of the orifice is usually not constant, so that accurate adjustment of flow rate and pressure is impossible. Wearing of screw-threads is also a great problem which is difficult to solve.
On the other hand, a flow control valve has been developed in order to solve the above mechanical problems, which is shown, for example, in FIG. 1. This valve includes a valve housing 1 having an inlet 1a for a pressurized fluid, a valve seat member 2 having a substantially conical inner surface valve seat 2a and fixed into the inlet 1a of the valve housing 1, a valve body 3 adapted to be in line contact with the conical inner surface valve seat 2a, and a throttle adjusting plate 4 shiftable in axial directions of the valve seat member 2 to shift the valve body 3, thereby controlling the flow rate of the pressurized fluid flowing between the valve seat 2a and the valve body 3. In order to shift the throttle adjusting plate 4, made of a magnetic material, in axial directions of valve seat member 2, an electromagnet 5 is arranged about the valve seat member 2 in opposition to and spaced apart from the throttle adjusting plate 4. The magnetic force of the electromagnet 5 is changed to vary the attractive force acting upon the throttle adjusting plate 4. In other words, by changing the electric current or voltage to be applied to a coil of the electromagnet 5 a force for shifting the valve body toward the valve seat (the force is referred to as "throttle force", hereinafter) can be continuously adjusted. In this manner, the valve is easily operated without causing the mechanical problems above described such as the play at screw-threads.
With the magnetic flow control valve above described, however, the throttle adjusting plate tends to go into violent free movement by axial and/or self-oscillation of the throttle adjusting plate, so that the plate would collide against the spherical valve body 3 or the valve body would collide against the valve seat. As a result, noise often occurs in pressure waveforms of the pressurized fluid exhausted from an outlet 1b formed in the valve housing 1. This is particularly acute in a servo valve when changing its set pressure, for example, shown in FIG. 2, wherein two magnetic flow control valves 20a and 20b are electrically connected through voltage control means 21 which serves to change the voltages applied to the valves 20a and 20b, in a manner one voltage increases and the other decreases so as to adjust throttle forces f.sub.1 and f.sub.2 to change a set pressure P.
In order to prevent the self-oscillation of the throttle adjusting plate and the spherical valve body due to so so-called "dancing movement", a clearance between the throttle adjusting plate 4 and a cover 6 may be as small as possible. However, such a small clearance therebetween simultaneously makes small a movable stroke of the valve body so as to reduce the controlled flow rate, thereby lowering the response of the device as a servo valve. That is particularly acute in case of a valve having a spherical valve body whose diameter is more than 5 mm.